A Conversion of Sample Medium from Water to Acetonitrile by Subzero Temperature Liquid-liquid Extraction for Acetonitrile-salt Stacking in Capillary Electrophoresis

Abstract

In capillary electrophoresis (CE) analysis, acetonitrile-salt stacking (ASS) is an efficient way for enhancing the response signal of the analyte which is dissolved in the aqueous medium of acetonitrile and NaCl. The higher volume fraction of acetonitrile (60 to 80%) in the sample medium is strictly demanded if ASS is conducted. Consequently, it is necessary to develop a convenient method for converting the water-based sample to the acetonitrile-based sample without any dilution of the analyte during the sample preparation procedure. Acetonitrile subzero temperature liquid-liquid extraction (ASTLLE) is the best choice for this purpose. ASTLLE is based on the unique property of acetonitrile that a phase separation can occur if the homogenous mixture solution of acetonitrile and water is kept at a subzero temperature for a short period. The upper layer is the acetonitrile-rich phase, and the lower layer is the water phase. Accompanying the phase separation, the analytes of relatively weak polarity such as mononitrophenol isomers (at acidic condition) can be transferred to acetonitrile-rich phase. The volume fraction of acetonitrile in the formed acetonitrile-rich phase was estimated on the basis of its viscosity which was determined by CE instrument. It was confirmed that the volume fraction of acetonitrile in the formed acetonitrile-rich phase was in the range of 70 – 80% (v/v), indicating that the sample processed by ASTLLE met the ASS demand for high volume fraction of acetonitrile. The combination of ASTLLE and ASS was successfully applied to the analysis of mononitrophenol isomers in acidic aqueous solution. The precisions (RSD) of the migration time and peak area for six replicate runs of the three mononitrophenol isomers were in the range of 1.1∼1.3% and 5.5∼8.5% respectively. The lowest detection limit of mononitrophenol isomers was 1×10-4μg-mL-1.The recovery ranged from 96.1 to 113.4%